1. Field of the Invention
The present invention relates to inverters. More specifically, the present invention relates to methods and apparatus for high speed digital controlled inverters for generating an accurate sine-wave from an input voltage.
2. Background
Inverters are incorporated into power supplies and are employed to transform an input voltage to a synthesized alternating current (a.c.) output voltage. A main objective in inverter design is to maintain accuracy of the synthesized a.c. voltage at the output terminal. Without the accuracy, loads sensitive to voltage, frequency variation and distortion will not function properly.
Several techniques of inverter design are known in the prior art. One of these techniques is known as the resonant approach. In the resonant approach, a direct current (d.c.) input signal or a rectified a.c. input signal is inverted to a new a.c. output signal which is generally isolated from the input signal. An objective of the resonant approach to inverter design is to satisfy the minimum distortion requirement for distortion sensitive loads.
In order to satisfy the minimum distortion requirement, a resonant tank circuit is employed. The quality (Q) factor of the tank circuit is selected to be high to control the amount of distortion in the synthesized a.c. output signal. However, this requirement necessitates that all inverter circuits have a high Q-factor. It is difficult to maintain voltage regulation of a high Q-factor circuit. Further, the delay time to register a change at the inverter input terminal and to compensate for a variation in the inverter output terminal (e.g., the load) is increased. Thus, the feedback loop in an inverter employing a resonant tank circuit is slowed by the high Q-factor circuits.
A second technique of inverter design of the prior art is known as the stepped approximation approach. In the stepped approximation approach, a series of different power drivers are assembled. In this synthesizing procedure, the signal is separated into a series of approximations in a power amplifier and thereafter the signal amplitudes are algebraically summed in a magnetic to limit the distortion in the synthesized a.c. output signal. The series of approximations may include sixteen steps of which eight of the steps are directed to the positive half sine-wave (e.g., 0 to .pi. radians) and the remaining eight steps are directed to the negative half sine-wave (e.g., .pi. to 2.pi. radians).
Thereafter, the positive half cycle is chopped into eight steps between (0-.pi.) radians. The eight steps are divided into three portions referred to as the minor, intermediate and major portions. The minor portion includes the first three steps in the sine-wave and delivers only a small amount of power. The intermediate portion includes the middle two steps comprising the top flat of the positive half cycle. The major portion includes the final three steps in the positive half cycle of the sine-wave and delivers the most power. The result is that three separate power amplifiers are created with each delivering a portion of the synthesized signal power. The output signals of the three separate power amplifiers are directed to a power transformer which includes three separate sets of windings. The triggered transformer windings enable the three transformer signals to be magnetically summed to provide the positive half cycle of the synthesized a.c. output signal.
The negative half cycle of the sine-wave is then likewise chopped into eight steps between (.pi.-2.pi.) radians. The eight steps are similarly divided into the minor, intermediate and major portions. Three separate power amplifiers are formed each providing a portion of the synthesized output signal. The separate signals are directed to the power transformer where the three triggered transformer windings enable the three transformer signals to be magnetically summed to provide the negative half cycle of the synthesized a.c. output signal.
The distortion in the synthesized sine-wave is approximately (5-10) % which too high for sensitive equipment such as a gyro. In order to minimize the distortion, the sine-wave is divided into a larger number of steps. This is accomplished by adding additional power stages. However, the larger number of steps limits the speed of the transient response. Thus, voltage regulation is slow resulting in instability for non-linear loads. Further, the additional power stages contribute to increased manufacturing costs and excessive weight. Variations of the stepped approximation approach are known but require overdesigning the inverter to adequately supply a non-linear load. Unfortunately, these alternative designs also experience limited transient response, instability, excessive distortion and increased cost and weight.
Thus, a need remains in the art for an improvement in conventional inverter design for providing a synthesized a.c. output signal.